Sn doped ZnS nanowires for white light source material

ABSTRACT

According to exemplary embodiments, a method of synthesizing tin (Sn)-doped Zinc Sulfide (ZnS) nanostructures for electroluminescent white light source includes coating a substrate, including a silicon oxide layer, with Sn by vacuuming depositing Sn as catalyst nanostructures on the substrate, placing the substrate coated with Sn in a furnace, introducing a carrier flow gas into the furnace, adding a ZnS power to the furnace, growing ZnS nanostructures, and dissolving Sn in the growing ZnS nanostructures. The S vacancies are on a surface of the ZnS nanostructures. The ZnS nanostructures are grown on the substrate having a temperature in a range of 750° C. to 850° C.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.15/683,989, filed on Aug. 23, 2017, and claims priority from and thebenefit of Pakistan Provisional Application No. 520/2016, filed on Aug.24, 2016, which is hereby incorporated by reference for all purposes asif fully set forth herein.

BACKGROUND Field

Exemplary embodiments relate to a method using vapor liquid solid (VLS)for the synthesis of nanostructures for an electroluminescent source.More particularly, exemplary embodiments relate to the synthesis of tin(Sn) self doped Zinc Sulfide (ZnS) nanostructures for electroluminescentwhite light source.

Discussion of the Background

White light is produced by combining, in a proper combination, threebasic colors, i.e., red (R), green (G), and blue (B). Based on theproportional weight of red or blue, the light source can be labeled aswarm or cool. Presently, there are two ways of producing white light, byhaving three LEDs (RGB) as three sources of colors and using blue LEDand two phosphor coated materials. Both techniques have their advantagesand disadvantages. Economic cost is a large issue related to white lightsources. So far, enormous efforts are being made to develop eithertechnology or fabricate a material that can produce white light at lowcost.

ZnS is a wide band gap material with an energy gap of 3.7 electron volt(eV). Crystal structures of ZnS can have two major structural defects,i.e., Zn vacancies and S vacancies. The two vacancies lead to formationof defect energy states in the band gap that are highly luminescent. Svacancies lead to emission blue color at 440 nm and Zn vacancies lead toemission of green wavelength at 520 nm, which are well established inthe art. Due to the dominance of Zn vacancies, ZnS is being used asgreen phosphor material or green electroluminescence material. However,if ZnS is doped with either Au, Mn, Ga or Sn. A third low energy bandmay be introduced into the band gap, which may lead to emission of redwavelength light, i.e., in 600-650 nm range. The amount of Zn and Svacancies has been reported to depend strongly on the choice ofcatalysts, growth, and post growth processing conditions. Incorporationof metal ions may provide an efficient radiative channel by introducingdefect states in the middle of band gap. Introduction of extrinsicdefects also alter the balance of various optically active statesparticipating in the recombination processes and thus substantiallymodify the radiative recombination channels and kinetics in the hostmaterial. Thus, the choice and location of the metal ion in the hostlattice is very important for defining the radiative recombinationpathways.

The above information disclosed in this Background section is only forenhancement of understanding of the background of the inventive concept,and, therefore, it may contain information that does not form the priorart that is already known in this country to a person of ordinary skillin the art.

SUMMARY

Under proper growth conditions, properly doped ZnS nanowires may emitall three fundamental colors to produce white light. In the presentembodiments, synthesis of ZnS nanowires using Sn as a catalyst dopantthat under certain growth conditions produces all three colors from asingle material (ZnS) is demonstrated.

Additional aspects will be set forth in the detailed description whichfollows, and, in part, will be apparent from the disclosure, or may belearned by practice of the inventive concept.

According to exemplary embodiments, a method of synthesizing tin(Sn)-doped Zinc Sulfide (ZnS) nanostructures for electroluminescentwhite light source includes coating a substrate, including a siliconoxide layer, with Sn by vacuuming depositing Sn as catalystnanostructures on the substrate, placing the substrate coated with Sn ina furnace, introducing a carrier flow gas into the furnace, adding a ZnSpower to the furnace, growing ZnS nanostructures, and dissolving Sn inthe growing ZnS nanostructures. The S vacancies are on a surface of theZnS nanostructures. The ZnS nanostructures are grown on the substratehaving a temperature in a range of 750° C. to 850° C.

The foregoing general description and the following detailed descriptionare exemplary and explanatory and are intended to provide furtherexplanation of the claimed subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the inventive concept, and are incorporated in andconstitute a part of this specification, illustrate exemplaryembodiments of the inventive concept, and, together with thedescription, serve to explain principles of the inventive concept.

FIGS. 1A and 1B are a device designs for ZnS:Sn based electroluminescentdevices for (a) current based, and (b) tunnel based sandwich devices.

FIGS. 2A, 2B, and 2C are the X-ray photoelectron spectroscopy (XPS) dataof the ZnS:Sn catalyzed and doped nanostructures showing Zn-2p, Sn-3d,and S-2p peaks.

FIGS. 3A, 3B, 3C, and 3D are room temperature PL spectra from the ZnSnanostructures doped with Sn grown at different temperatures.

The Figures also show fluorescent images of same samples showing thedifference in emission patterns.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

In the following description, for the purposes of explanation, numerousspecific details are set forth in order to provide a thoroughunderstanding of various exemplary embodiments. It is apparent, however,that various exemplary embodiments may be practiced without thesespecific details or with one or more equivalent arrangements. In otherinstances, well-known structures and devices are shown in block diagramform in order to avoid unnecessarily obscuring various exemplaryembodiments.

In the accompanying figures, the size and relative sizes of elements maybe exaggerated for clarity and descriptive purposes. Also, likereference numerals denote like elements.

For the purposes of this disclosure, “at least one of X, Y, and Z” and“at least one selected from the group consisting of X, Y, and Z” may beconstrued as X only, Y only, Z only, or any combination of two or moreof X, Y, and Z, such as, for instance, XYZ, XYY, YZ, and ZZ. As usedherein, the term “and/or” includes any and all combinations of one ormore of the associated listed items.

Although the terms “first,” “second,” etc. may be used herein todescribe various elements, components, and/or regions, these elements,components, and/or regions should not be limited by these terms. Theseterms are used to distinguish one element, component, and/or region fromanother element, component, and/or region. Thus, a first element,component, and/or region discussed below could be termed a secondelement, component, and/or region without departing from the teachingsof the present disclosure.

The terminology used herein is for the purpose of describing particularembodiments and is not intended to be limiting. As used herein, thesingular forms, “a,” “an,” and “the” are intended to include the pluralforms as well, unless the context clearly indicates otherwise. Moreover,the terms “comprises,” “comprising,” “includes,” and/or “including,”when used in this specification, specify the presence of statedfeatures, integers, steps, operations, elements, components, and/orgroups thereof, but do not preclude the presence or addition of one ormore other features, integers, steps, operations, elements, components,and/or groups thereof.

Design:

An electroluminescent (EL) device consisting of ZnSnanowires/nanostructures doped with suitable concentration of Sn asphosphor material in the device that works on the principle ofexcitation through current flow (FIG. 1A) in the device or appliedelectric field in a sandwich structure (FIG. 1B). There are three strongluminescent centers in the Sn doped ZnS nanostructures, namely, Svacancies responsible for the blue band, Zn vacancies for the green bandand Sn^(2+/4+) states for the red band emission. Electrons areaccelerated through the nanowires of ZnS and excite carriers from thevalence band to the defect states through the conduction band. Theexcited carriers relax to the defect states (S, Zn vacancies andSn^(2+/4+) ionized dopants), producing luminescence.

Synthesis:

ZnS nanowires may be synthesized on Si (100) substrates with a thin (2-3nm) native oxide layer and a Sn catalyst. Ultra-thin film (2 nm thick)of Sn may be deposited on Si substrates by thermal evaporation at roomtemperature at a base pressure of 3×10⁻⁶ Torn Prior to deposition,substrates may be ultrasonically cleaned. The Sn coated substrates maythen be placed in a single zone horizontal tube furnace along withprecursor (ZnS) powder in an alumina boat. The precursor material may beplaced at the center of the tube and multiple substrates may be placedin downstream and away from the precursor. The furnace may be heated to1000° C. in a continuous flow of a mixture of hydrogen (H₂) (5%) andnitrogen (N₂) (95%) gases at 20 standard cubic centimeter per minute(Sccm). Measured substrate temperatures may be 850° C., 800° C., 780°C., and 750° C. Growth may be performed for 2 hours. Finally synthesizedZnS nanostructures may be furnace cooled under a flowing N₂ atmosphere.

Optical Characterization:

FIGS. 3A, 3B, 3C, and 3D show the room temperature photoluminescencespectra and corresponding fluorescence microscopy images of ZnSnanowires synthesized using Sn as a catalyst at various growthtemperatures. The fluorescence images may be obtained by exciting with aUV lamp. The photoluminescence spectra may be obtained at roomtemperature by exciting with 325 nm He—Cd laser and normalized to thespectrometer and detector response.

The growth temperature showed profound effect on the photoluminescenceproperties of ZnS nanowires. This was due to a play between theintrinsic defects and doping of catalyst during growth, which wasdifferent at different growth temperatures. The results shown in FIGS.3A, 3B, 3C and 3D demonstrate that the photoluminescence spectra changedits lineshape in nanowires grown at different temperatures. At hightemperature growth, Sn defects were substantially dominant, but as thegrowth temperature was reduced, the balance between intrinsic andextrinsic defects changed and in ZnS nanowires grown at 780° C., abroadband spectrum was observed between 400 nm to 700 nm. In ZnSnanowires grown at lower temperatures, the balance was again disturbedand green and red luminescence was dominant. It was observed that blueemission due to S vacancies may be sensitive to various factors,including, surface to volume ratio and incorporation of Sn as a dopant.

The presence of three fundamental colors show the Sn doped ZnS nanowiresas a promising material for white light source. This also introduces thepossibility to tune various colors from a single source. The role of Sndoping in the optical properties of ZnS nanowires was shown when Sndoping in ZnS nanostructures produces more relaxed S vacancy ZnSstructures at the surface, responsible for blue light emission. This wasobserved that only in a very small window of growth conditions, Svacancies dominate and hence become dominant for blue emission. Thismakes Sn doped ZnS nanostructures ideal to fabricate multi-color lasersor LEDs.

FIGS. 2A, 2B, and 2C show the XPS data collected during the visit fromone sample of ZnS doped with Sn at 780° C. It shows significant amountof Sn present, which was also apparent from the photoluminescencespectra.

Photoluminescence Spectroscopy

Room temperature photoluminescence spectroscopy was performed to observethe emission spectrum of ZnS:Sn nanostructures grown at differenttemperatures shown in FIGS. 3A, 3B, 3C, and 3D.

As shown in FIG. 3A, ZnS:Sn grown at 850° C. had mainly red emission ina relatively sharp band centered at 650 nm, which may be attributed tothe presence of overwhelming number of Sn^(2+/4+) states. Absence ofemission bands at 440 nm and 520 nm may show nonexistence of S and Znvacancies, respectively. This may be clearly observed in the fluorescentimage shown with the spectra.

As shown in FIG. 3B, the photoluminescence spectra obtained from ZnS:Sngrown at 800° C. had a much broader band from 500 nm to 670 nm, and mayconsist of two contributions, i.e., emission from Zn vacancy states andSn^(2+/4+) states. The fluorescent image also showed green lightcontribution.

As shown in FIG. 3C, the photoluminescence spectra from ZnS:Snsynthesized at 780° C. had the broadest band starting from 400 nm to 700nm with three distinct broader peaks. The peaks may be observed atapproximately 440 nm, 540 nm, and 630 nm. The fluorescent image alsoshowed much brighter spots.

As shown in FIG. 3D, the photoluminescence spectra from ZnS:Sn grown at730° C. may also show a broad band, but the contribution from 440 nmstates may be negligible or very minute. It may have contributions fromthe green and red emission states. This may be reflected in thefluorescent image.

The photoluminescence measurements may confirm the following:

The presence and number of vacancy states strongly depend on the growthtemperature, S vacancies were most sensitive to the growth temperature,and S vacancies were present on the surface of nanostructures.

In order to have substantial blue emission (due to S vacancies), thewindow of growth of ZnS:Sn nanostructures may be very small. The bestresults may come from ZnS:Sn nanowires grown at 780° C.

Although certain exemplary embodiments and implementations have beendescribed herein, other embodiments and modifications will be apparentfrom this description. Accordingly, the inventive concept is not limitedto such embodiments, but rather to the broader scope of the presentedclaims and various obvious modifications and equivalent arrangements.

What is claimed is:
 1. A method of synthesizing tin (Sn)-doped ZincSulfide (ZnS) nanostructures for electroluminescent white light source,comprising: coating a substrate, comprising a silicon oxide layer, withSn by vacuuming depositing Sn as catalyst nanostructures on thesubstrate; placing the substrate coated with Sn in a furnace;introducing a carrier flow gas into the furnace; adding a ZnS powder tothe furnace; growing ZnS nanostructures; and dissolving Sn in thegrowing ZnS nanostructures; wherein S vacancies are on a surface of theZnS nanostructures, wherein the ZnS nanostructures are grown on thesubstrate having a temperature in a range of 750° C. to 850° C., whereinthe Sn self-doped ZnS nanostructures have broad emission in a range of400 nm to 650 nm, wherein the deposited Sn is 2 nm or less, and whereinthe ZnS nanostructures doped with Sn are grown on the substrate having atemperature of 780° C.
 2. The method of claim 1, wherein blue, red, andgreen emissions of the ZnS nanostructures are controlled by growthconditions of Sn doped ZnS nanostructures.
 3. The method of claim 1,wherein the carrier flow gas is a mixture of hydrogen and nitrogen gasis in a ratio of 1:4 that is introduced at flow rate of 30 Sccm.